The present invention relates to systems for controlling the recirculation of fuel vapor from the vapor dome in a tank to the filler tube for controlling operation of the automatic nozzle shutoff when the level of fuel in the tank reaches a predetermined maximum.
Fuel vapor recirculation systems have been employed in motor vehicle fuel tank installations for accommodating the reduced pressure in the region surrounding the liquid discharge from the dispensing nozzle during refueling and particularly where a mechanical seal is provided about the nozzle upon insertion in the filler tube for preventing fuel vapors escaping to the atmosphere during refueling.
In such systems problems have been encountered in designing the system to accommodate the wide variation in nozzle dispensing rates experienced in different regions. For example, in the majority of vehicle service regions the nozzles are set to shut off when a vacuum of about 1.75 to 2.5 kPa is reached in the filler tube. The normal vacuum during fuel discharge is about 0.5 kPa and requires about six liters per minute of recirculation vapor flow to maintain the vacuum at this level. However, in regions, a greater degree of sealing is required about the nozzle resulting in a need for about 70 liters per minute recirculation vapor flow to maintain 0.5 kPa vacuum and to prevent premature nozzle shut-off. Thus, it has been difficult to design a system for providing sufficient vapor recirculation to the filler tube during refueling in order to accommodate the wide range of vapor flow required to accommodate the variety of refueling requirements. If the system is designed to accommodate the maximum vapor flow to be required during refueling, the system will react to cause pressure build up in the filler tube in the region surrounding the nozzle and result in leakage of fuel vapor past the nozzle seal to the atmosphere. It will be understood that such systems include a second vapor line to a vapor storage device, such as a canister filled with granulated charcoal, for storing fuel vapor displaced by the rising liquid fuel level in the tank. The system must therefore maintain a proper balance of vapor flow to the canister and to the filler tube regions surrounding the nozzle in order to maintain the integrity of the system in preventing escape of fuel vapor to the atmosphere and also to facilitate proper filling from the refueling nozzle and prevent premature activation of the automatic shutoff device provided in the nozzle.